Electrochemiluminescent (ECL) methods and systems are useful in a variety of applications including medical diagnostics, food and beverage testing, environmental monitoring, manufacturing quality control, drug discovery and basic scientific research. There are a number of commercially available instruments that utilize ECL for analytical measurements.
Species that can be induced to emit ECL (ECL moieties) have been used as ECL labels in various testing procedures. The light generated by ECL labels can be used as a reporter signal in diagnostic procedures (Bard et al., U.S. Pat. No. 5,238,808). For instance, an ECL label can be covalently coupled to a binding agent such as an antibody or a nucleic acid probe, and the participation of the binding reagent in a binding interaction can be monitored by measuring ECL emitted from the ECL label. Alternatively, the ECL signal from an ECL-active compound can be indicative of the chemical environment (see, e.g., U.S. Pat. No. 5,641,623 which describes ECL assays that monitor the formation or destruction of ECL coreactants).
For more background on ECL, ECL labels, ECL assays and instrumentation for conducting ECL assays see U.S. Pat. Nos. 5,093,268; 5,147,806; 5,324,457; 5,591,581; 5,597,910; 5,641,623; 5,643,713; 5,679,519; 5,705,402; 5,846,485; 5,866,434; 5,786,141; 5,731,147; 6,066,448; 6,136,268; 5,776,672; 5,308,754; 5,240,863; 6,207,369; and 5,589,136 and Published PCT Nos. WO99/63347; WO00/03233; WO99/58962; WO99/32662; WO99/14599; WO98/12539; WO97/36931 and WO98/57154.
Commercially available ECL instruments are widely used for reasons including their excellent sensitivity, dynamic range, precision, and tolerance of complex sample matrices. Many commercially available instruments use flow cell-based designs with permanent reusable flow cells. The use of a permanent flow cell provides many advantages but also some limitations, for example, in assay throughput. In some applications, for example, the screening of chemical libraries for potential therapeutic drugs, assay instrumentation should perform large numbers of analyses at high speeds on small quantities of samples.
A variety of techniques have been developed for increasing assay throughput and decreasing sample size. The use of multi-well assay plates allows for the parallel processing and analysis of multiple samples distributed in multiple wells of a plate. Typically, samples and reagents are stored, processed and/or analyzed in multi-well assay plates (also known as microplates or microtiter plates). Multi-well assay plates can take a variety of forms, sizes and shapes. For convenience, some standards have appeared for some instrumentation used to process samples for high throughput assays. Multi-well assay plates typically are made in standard sizes and shapes and with standard arrangements of wells. Some well established arrangements of wells include those found on 96-well plates (12×8 array of wells), 384-well plates (24×16 array of wells) and 1536-well plate (48×32 array of wells). The Society for Biomolecular Screening and ANSI have published microplate specifications for a variety of plate formats (see, http://www.sbsonline.org).
There is a need for ECL assay systems, and assays systems based on other electrochemical methods, that require lower sample volume, and are less expensive, faster, and more sensitive. As these assays move to the nanoscale to address these needs, it is increasingly difficult to separate the working electrode from the counter electrode: As the working and counter electrodes are brought closer together in the same cell, undesirable redox byproducts formed at the counter electrode can interact with species at the working electrode.
To date, cells using one capacitive and one faradaic electrode have been used in solid state systems, for example, to inject charge into thin layers of luminescent organic polymers to aid the observation of spectroscopic properties. In such systems, one electrode contacts the polymer layer and the other electrode is a small tip separated from the polymer layer by a ˜10 nm insulating layer of impurities or air. See for example, Adams, et al., J. Phys. Chem. B, 2000, 104, 6728. These cells are not electrochemical cells, do not use an electrolyte solution, and are not designed to contain an electrolyte solution. Configurations involving a capacitive electrode and a reference electrode have been used to examine double layer effects. See, for example, Grahame, Chem. Rev., 1947, 41, 441. These studies focus on the importance of the polarized electrode. Any faradaic effects at the reference electrode were not of interest and were neglected.
There remains a need for an electrochemical apparatus that reduces the introduction of undesirable electrochemically generated byproducts into the sample.
There remains a need for an electrochemical apparatus that separately controls the timing of the generation of oxidation and reduction products.